The documents cited in this specification are as follows, and these documents are cited based on their document numbers. Document 1: Japanese Patent Application Laid-Open No. 10-207580. The document 1 describes a voltage-monitoring type power on reset circuit (for example, FIG. 2) mounted in a contactless IC card which is operated by the operating voltage formed by rectifying the alternate current (AC) supplied from outside. The signal outputted from this power on reset circuit is used for the reset of the microcomputer mounted in the IC card.
The so-called contactless IC card in which a semiconductor integrated circuit device and an antenna are provided exchanges the information between an interrogator and the semiconductor integrated circuit device, and it is used to achieve various functions such as the transmission of the data held by the contactless IC card and the storage of the data transmitted from the interrogator. The semiconductor integrated circuit device mounted in the contactless IC card receives a high frequency signal supplied from the interrogator by the antenna mounted in the contactless IC card and rectifies and smoothes the voltage generated at both ends of the antenna to form an internal voltage necessary for the operation of the internal circuit. In this case, when excessive power is supplied from the interrogator and the power supply voltage higher than the withstand voltage of the devices constituting the internal circuit is supplied, the devices are broken. For its prevention, a control circuit for monitoring the power supply voltage level generated inside is provided in many cases so as not to supply the power supply voltage higher than the withstand voltage of the devices.
Meanwhile, the signal processing circuit mounted in the semiconductor integrated circuit device in the contactless IC card detects the power supply voltage level generated as described above, and if the power supply voltage reaches a level where the signal processing circuit mounted in operates without malfunction, the signal processing circuit is operated and if the power supply voltage does not reaches a level where the signal processing circuit operates without malfunction, the signal processing circuit is stopped. As an example of this type, the document 1 discloses a signal processing circuit which detects the power supply voltage level and if the power supply voltage is lower than a predetermined voltage level, the signal processing circuit is transitioned to a reset state and stopped and if the power supply voltage is higher than the predetermined voltage level, the reset state is released to operate the signal processing circuit.
Prior to this application, the inventors of the present invention have examined the problems caused when the semiconductor integrated circuit device provided with the voltage-monitoring type reset circuit according to the document 1 is used in the contactless IC card.
FIG. 1 shows the current (I)-voltage (V) characteristics VL examined by the inventors of the present invention, which are outputted from a rectifier circuit in the case where power is supplied in the form of electromagnetic wave from the interrogator and the rectifier circuit is connected to an antenna provided in the contactless IC card. In this case, the characteristics VL are represented by the relational expression of V=Vo−I·Rout. This means that the voltage at both ends of the antenna is changed depending on the current passing through the load connected to the output terminal of the rectifier circuit and is equivalent to the voltage source having the output impedance Rout. Vo is the voltage when it is assumed that no current passes through the load, and the supply voltage V is reduced when current passes through the load. At this time, the slope of the current-voltage characteristics VL is equal to the output impedance Rout.
FIGS. 2A and 2B show an example of an operating waveform in the case where the semiconductor integrated circuit device provided with the reset circuit according to the document 1 is connected to the interrogator and the antenna with the current-voltage characteristics VL shown in FIG. 1. The power on reset circuit exemplified in FIG. 2 of the document 1 generates the power on reset signal by comparing the voltage divided by the voltage divider for dividing a power supply voltage and the reference voltage from the bandgap reference circuit. By doing so, when the power supply voltage is higher than a predetermined voltage level VACT, the reset state of the signal processing circuit is released and the signal processing circuit starts to operate.
The reset signal outputted from the reset circuit of the document 1 is checked at the time T0 in FIG. 2 before the signal processing circuit starts to operate, and if the power supply voltage is higher than the predetermined voltage level VACT, the signal processing circuit is operated between the time T1 and time T2. In FIG. 2, since the power supply voltage is higher than the predetermined voltage level VACT at the time T0, the signal processing circuit mounted inside is permitted to operate and the signal processing circuit starts to operate. Therefore, the consumption current when the signal processing circuit is being operated is increased by I1 in comparison to that when it is not operated.
If the output impedance Rout is 0Ω, the change in power supply voltage level due to the change in consumption current does not occur, and the signal processing circuit can be operated by the power supply voltage higher than the predetermined power supply voltage level VACT. However, when the output impedance Rout is high, the power supply voltage VDD drops by the product of the change in consumption current and the output impedance Rout. Therefore, since the power supply voltage lower than the predetermined power supply voltage level VACT by the product of the change in consumption current and the output impedance Rout is supplied to the signal processing circuit, the signal processing circuit is operated by the voltage lower than the predetermined power supply voltage level. Consequently, there is the possibility that the characteristics of the signal processing circuit are changed and the malfunction due to the characteristic degradation occurs.
An object of the present invention is to provide a semiconductor integrated circuit device having a function to output a reset signal for operating a signal processing circuit at a predetermined power supply voltage even when the current is changed due to the operation of the signal processing circuit.